Erosion rate study at the Allchar deposit (Macedonia) based ...tively. In this context, the knowledge of the erosion rate of the overburden rocks since the formation of lor-andite,

RESEARCH ARTICLE10.1002/2015GC006054

Erosion rate study at the Allchar deposit (Macedonia) based onradioactive and stable cosmogenic nuclides (26Al, 36Cl, 3He,and 21Ne)M. K. Pavicevic1, V. Cvetkovic2, S. Niedermann3, V. Pejovic4, G. Amthauer1, B. Boev5, F. Bosch6,I. Anicin4, and W. F. Henning7

1Chemistry and Physics of Materials, University of Salzburg, Salzburg, Austria, 2Faculty of Mining and Geology, Universityof Belgrade, Belgrade, Serbia, 3Helmholtz-Zentrum Potsdam—Deutsches GeoForschungsZentrum GFZ, Potsdam,Germany, 4Faculty of Physics, University of Belgrade, Belgrade, Serbia, 5Faculty of Mining and Geology, University of �Stip,�Stip, Macedonia (FYROM), 6Gesellschaft f€ur Schwerionenforschung GSI, Darmstadt, Germany, 7Argonne NationalLaboratory, Physics Division, Argonne, Illinois, USA

Abstract This paper focuses on constraining the erosion rate in the area of the Allchar Sb-As-Tl-Audeposit (Macedonia). It contains the largest known reserves of lorandite (TlAsS2), which is essential for theLORanditeEXperiment (LOREX), aimed at determining the long-term solar neutrino flux. Because the erosionhistory of the Allchar area is crucial for the success of LOREX, we applied terrestrial in situ cosmogenicnuclides including both radioactive (26Al and 36Cl) and stable (3He and 21Ne) nuclides in quartz, dolomite/calcite, sanidine, and diopside. The obtained results suggest that there is accordance in the values obtainedby applying 26Al, 36Cl, and 21Ne for around 85% of the entire sample collection, with resulting erosion ratesvarying from several tens of m/Ma to �165 m/Ma. The samples from four locations (L-8 CD, L1b/R, L1c/R,and L-4/ADR) give erosion rates between 300 and 400 m/Ma. Although these localities reveal remarkablyhigher values, which may be explained by burial events that occurred in part of Allchar, the erosion rateestimates mostly in the range between 50 and 100 m/Ma. This range further enables us to estimate thevertical erosion rate values for the two main ore bodies Crven Dol and Centralni Deo. We also estimate thatthe lower and upper limits of average paleo-depths for the ore body Centralni Deo from 4.3 Ma to thepresent are 250–290 and 750–790 m, respectively, whereas the upper limit of paleo-depth for the ore bodyCrven Dol over the same geological age is 860 m. The estimated paleo-depth values allow estimating therelative contributions of 205Pb derived from pp-neutrino and fast cosmic-ray muons, respectively, which isan important prerequisite for the LOREX experiment.

1. Introduction

1.1. The LOREX ProjectThe Allchar Sb-As-Tl-Au hydrothermal deposit is related to a Pliocene volcano-intrusive complex located at thenorth-western margins of the Ko�zuf Mountains and close to the border between the Former Yugoslav Repub-lic of Macedonia and Greece [Frantz et al., 1994; Jankovic and Jelenkovic, 1994]. Allchar is a world-class thalliumdeposit unique by its quantity of thallium reserves and by the number of thallium minerals discovered. Itcontains sufficient quantities of lorandite (TlAsS2), and because this mineral can be used as a geochemicaldetector of proton-proton (pp) solar neutrinos [Freedman et al., 1976], the deposit has gained the interest of awider community of astrophysicists, nuclear physicists, and geochemists [e.g., Frantz et al., 1994].

The main goal of the LOREX (LORanditeEXperiment) Project is to determine the average fluence of solar pp-neutrinos Um for the age period of the lorandite in the Allchar deposit [Pavicevic, 1994], according to the fol-lowing equation:

Umrm5CðN205PbÞexp2ðN205PbÞB

mð12ekT Þ (1)

where C is a constant (C 5 3.79 3 10219 mol a21), rm is the cross section for the capture of solar (pp) neutri-nos by 205Tl, (N205Pb)exp is the experimentally determined number of 205Pb atoms in lorandite of mass m,

Key Points:� Erosion rate of a unique Tl-deposit is

studied by radioactive/stablenuclides� The vertical erosion rate is obtained

for the locations with most Tl-rich ore� The study contributes to determining

the long-term average flux of solarpp-neutrinos

Correspondence to:M. K. Pavicevic,[email protected]

Citation:Pavicevic, M. K., V. Cvetkovic,S. Niedermann, V. Pejovic,G. Amthauer, B. Boev, F. Bosch,I. Anicin, and W. F. Henning (2016),Erosion rate study at the Allchardeposit (Macedonia) based onradioactive and stable cosmogenicnuclides (26Al, 36Cl, 3He, and 21Ne),Geochem. Geophys. Geosyst., 17,doi:10.1002/2015GC006054.

Received 12 AUG 2015

Accepted 19 JAN 2016

Accepted article online 23 JAN 2016

VC 2016. American Geophysical Union.

All Rights Reserved.

PAVICEVIC ET AL. EROSION RATES AT A UNIQUE TL-DEPOSIT 1

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(N205Pb)B is the number of 205Pb atoms in lorandite resulting from background reactions, k is the decayconstant of 205Pb (k 5 4.68 3 1028 a21), and T 5 4.3 Ma is the age of the thallium mineralization in Allchar[Neubauer et al., 2009].

The number of 205Pb atoms (N205Pb)B (see equation (1)) produced by cosmic radiation, mostly by (l,pn)-reactions, and by natural nuclear reactions connected to U and Th decay, must be determined quantita-tively. In this context, the knowledge of the erosion rate of the overburden rocks since the formation of lor-andite, i.e., the paleo-depth as a function of time, is of utmost importance [Pavicevic, 1994]. This is why atthe beginning of LOREX, by the end of the eighties and the beginning of the nineties, we started withdetailed studies of the erosion rate in Allchar. These studies were twofold, focusing on geomorphologicalanalysis and on applying terrestrial in situ cosmogenic nuclides (hereafter TCN). The first TCN studies basedon 36Cl [Dockhorn et al., 1991] suggested a small erosion rate in this area, and therefore a high contributionof cosmic-ray-produced 205Pb, which would make further investigations on the LOREX project pointless.However, the subsequent measurements of 26Al concentrations suggested higher erosion rates [Pavicevicet al., 2004], what encouraged further research.

The aim of this paper is to provide an estimate of the erosion rate in the area of the Allchar deposit byapplying both radioactive (26Al and 36Cl) and stable (3He and 21Ne) cosmogenic nuclides in various minerals(quartz, dolomite/calcite, sanidine, and diopside). Although the obtained values may not reflect precise ero-sion rates in the studied region, they should provide a best approximation, i.e., an order of magnitude esti-mate of the prevailing erosion rate. Such estimates are sufficient for assessing the order of magnitude ofthe contribution of cosmic radiation in the 205Pb production during the last 4.3 Ma and thereby representan important step forward in demonstrating the feasibility of the LOREX Project. This is because, before con-tinuing with time-consuming and expensive analytical procedures, we need to know the order of magni-tude of the denudation rate.

1.2. Geological Outline of the Allchar AreaThe Allchar deposit is spatially and petrogenetically related to a Pliocene volcano-intrusive complex, whichis situated between the Pelagonian tectonic unit and the Vardar zone suture in the east and west, respec-tively [e.g., Karamata, 2006; Schmid et al., 2008]. The immediate volcanic basement consists of Triassic-Jurassic rock series, predominantly dolomite and dolomitic marble, limestone, sandstone, clayey schist,quartzite, and chert. Detailed information about the geology of Allchar can be found in Boev [1988], Frantzet al. [1994], Jankovic and Jelenkovic [1994], Karamata et al. [1994], and Amthauer et al. [2012], amongothers.

Three phases of magmatic activity are distinguished in the Allchar area. The first one took place in the Mio-cene (14.3–8.2 Ma), when numerous calc-alkaline dykes were emplaced. The second phase occurred in thePliocene, producing latite, quartzlatite, and trachyte to subordinate andesite/dacite subvolcanic intrusions.The third phase occurred between 6.5 and 1.7 Ma [Boev, 1988], and during this event the thallium minerali-zation also formed. The andesite lavas and tuffs of the area Crven Dol formed 6.5–3.9 Ma, the tuffs from thelocalities Vitacevo and Rudina formed from 5.1 6 0.1 to 4.31 6 0.02 Ma, whereas the Kojcov Rid latites origi-nated between 4.8 and 3.3 Ma [Neubauer et al., 2009]. Troesch and Frantz [1992] determined the geologicalage of a sanidine from adit Nr. 21 (Crven Dol) to be 4.22 6 0.07 Ma, which is in very good agreement withthe age of the sanidine from Rudina, i.e., 4.31 6 0.02 Ma [Neubauer et al., 2009]. The Allchar volcanic rocksare overlain by volcano-sedimentary rocks mainly represented by tuffaceous dolomite that was depositedin sublacustrine basins.

The Allchar Sb-As-Tl-Au deposit is a NNW-SSE stretching, 2 km long and 300–500 m wide zone whichcomprises several ore bodies. The mineralization is mainly localized along unconformities between thebasement rocks and the volcanic or volcaniclastic cover. The precipitation of the Sb-As-Tl-Au mineraliza-tion was dated by the Ar/Ar method at 4.31 6 0.02 Ma [Neubauer et al., 2009]. It was accompanied byintensive hydrothermal alteration predominantly represented by silicification and argilitization. The mainthallium ore bodies are Crven Dol and Centralni Deo, and they are situated along the contacts betweensubvolcanic andesites and their volcaniclastic counterparts and hydrothermally altered dolomites [Boevand Jelenkovic, 2012].

Geochemistry, Geophysics, Geosystems 10.1002/2015GC006054


There are no accurate data about active uplift in this part of the Balkan Peninsula. However, it is docu-mented that from the late Miocene onward, this area underwent mainly strike-slip tectonics [Dumurdzanovet al., 2005; Hoffmann et al., 2010].

Paleoclimatic reconstructions and the timing of glacial retreat in the Allchar area are poorly constrained.Namely, the data about these events are either very limited and lack quantitative measurements [Cvijic,1903, 1913], or they mostly refer to broad regional scales, sometimes encompassing almost the entireBalkan Peninsula [Menkovic et al., 2004]. Nevertheless, the existing studies suggest that although a signifi-cant part of the Balkan area was covered by ice during the Quaternary, it is very likely that the Last GlacialMaximum affected only the highest altitudes.

2. Sampling and Experimental Methods

2.1. Locations of SamplesThe Allchar area is mostly covered by vegetation, therefore only a few locations were suitable for sampling.All samples were ranging in weight from 2 to 5 kg and were taken from the surface to a depth of 10 cm,except for three calcite/dolomite samples from depths of 1.3 m (Do 50L/13), 2.5 m (Do 1/13), and 5 m (Do8/13).Over more than 10 years, we have been collecting samples that would primarily contain quartz that iscommonly used for studies of the cosmogenic radionuclides 10Be and 26Al. Figure 1 shows a map of 11 sam-pling locations, which are either near or above the lorandite ore bodies of Crven Dol and Centralni Deo.This ensures that the erosion rate estimates are significant for the determination of the paleo-depth of thethallium mineralization.

All the samples were studied petrologically and mineralogically, by applying optical microscopy, X-ray dif-fraction, and, in some cases, Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM-EDX). Information about the sampling locations and the petrography of samples is summarized in Table 1.

2.2. Mineral Separation Procedures2.2.1. Preparation of Quartz ConcentratesTotal quartz concentrates (60–70% pure silica) were produced from pristine rock samples (about 3–5 kgeach), applying standard mineral separation techniques [Pavicevic et al., 2006]: (I) crushing and grinding, (II)sieving to selected grain sizes <0.5 mm, and (III) separation of quartz concentrates on the shaking tablewith water. From the quartz concentrates, two samples of 80–100 g each were taken and were etched fol-lowing the procedure described in Pavicevic et al. [2004]. The etching was performed repeatedly (3–5 times)with 30% H2SiF6 and ‘‘aqua regia’’ (HCl:HNO3 5 3:1) at room temperature over a time period of 3–4 days,under constant shaking. After each step, the sample was washed with bidistilled water and dried at 808C,along with a control of weight loss. The quality of each treatment was checked with X-ray diffraction. At theend of the treatment, we obtained pure stoichiometric quartz with >99% SiO2 [Pavicevic et al., 2006].

The preparation procedure for hydrothermal quartz was similar to the one applied for total stoichiometricquartz. Relatively big (�5–15 mm) crystals of hydrothermal quartz were sampled at three locations (cf.Tables 4 and 5). 2–3 g of crystals of hydrothermal quartz were etched with 30% H2SiF6 and ‘‘aqua regia’’(HCl:HNO3 5 3:1). The quality of each treatment was checked with X-ray diffraction and SEM-EDXtechniques.2.2.2. Preparation of Calcite/Dolomite ConcentratesSamples of calcite/dolomite were taken at the localities L-1c/R, L-3c/RML, and L-8/CD (sample numbers: Do50L/13, Do 1/13, and Do 8/13) in quantities from 3 to 4 kg (Table 1). For producing calcite/dolomite concen-trates, a standard procedure of mineral separation was carried out. It involved crushing, sieving, and grind-ing to a fraction of <0.5 mm, then, separation on a shaking table and magnetic separation for reducingquartz concentrations. Because they contain both minerals, we labeled these samples calcite/dolomiteconcentrates.2.2.3. Preparation of Sanidine and DiopsideSanidine (K0.75Na0.25AlSi3O8) was separated from a hydrothermally altered and mineralized volcaniclasticrock (3–4 kg of a sample from the locality L-1a/R; Table 1). The classical procedure comprised grinding,crushing, and sieving (<125 mm to <2 mm), and the obtained fraction 400–630 mm was used for furthertreatment. Pure sanidine crystals (�500 mg) were separated by hand-picking using a binocular microscope.X-ray diffraction data (performed in duplicate) confirmed a sanidine crystal structure.



Figure 1. Right: A simplified geological map of the wider area of the Allchar deposit (modified from Jankovic and Jelenkovic [1994]). Detail A: Map of the sampling locations at the CrvenDol locality. Detail B: Sampling locations at the Centralni Deo locality. Gray circles: Positions of samples collected for analysis of cosmogenic nuclides.



A similar procedure was applied for diopside (MgCaSi2O6) separation. One sample of 2 kg from the localityL-7/KR (Table 1) underwent grinding, crushing, and sieving (<125 to <355 mm). The fraction 250–355 mmwas further separated using a shaking table. After that, pure diopside crystals (�370 mg) were separatedusing magnetic separation and hand-picking. X-ray diffraction and SEM-EDX analyses confirmed that theobtained separates consisted of >96% pure diopside.

2.3. Determination of Cosmogenic Nuclide Concentrations2.3.1. Measurements of 26Al in Total Stoichiometric QuartzFifteen samples of total stoichiometric quartz in quantities of ca. 20 g/sample were sent in two series to thePRIME Lab (Purdue Rare Isotope Measurement Laboratory, Purdue University) for Acceleration Mass Spec-trometry measurements of 26Al. Each sample was dissolved in 5:1 HF/HNO3 and after dissolution (no residueleft) an aliquot was taken for stable 27Al determination by Inductively Coupled Plasma-Optical EmissionSpectrometry. To the rest of the samples, the standard chemical procedures used at PRIME lab were applied,finally yielding targets in the form of Al2O3.For 26Al, the standard of Nishiizumi [2004] was used, which corre-sponds to a 26Al half-life of 708 6 17 ka.2.3.2. Determination of 36Cl in Calcite/DolomiteThe mineral composition of calcite/dolomite concentrates was characterized both qualitatively and quanti-tatively by X-ray powder diffraction including a Rietveld analysis. This investigation revealed that thesamples were predominantly represented by calcite (95.8–99.2%), whereas dolomite was subordinate (0.5–3.4%). The concentration of Ca and K contents was determined by Inductively Coupled Plasma Mass Spec-trometry (ICP-MS). This showed that K concentrations were very low (ca. 0.01%), indicating that 36Cl produc-tion from K is negligible. Similarly, the measured values of stable chlorine are low (mostly <0.5 ppm). This

Table 1. Rocks, Minerals, and Site Parameters for the Sampling Location Sites in Allchar

Location Sample Number Petrography and Mineral Composition Latitude Longitude Elevation (m) Slope (8)

L-7/KR Di/09 Slightly altered volcaniclastic rock; Plagioclase, biotite,quartz, christobalite, sanidine, diopside, albite

418100500 21857034.300 965 5

L-8/CD Do 8/13 Fine-grained dolomitic limestone with stylolitictextures

4189033.400 2185703100 821 20

L-1a/R Sa/08 Hydrothermally altered dolomite; Hydrothermalquartz, pyrite, marcasite, sanidine, Fe-hydroxides,jarosite, K-feldspar

41809014.200 21857034.800 855 35

L-1b/R 4b/2 Hydrothermally altered and mineralized volcaniclasticrock with abundant quartz veinsHydrothermal quartz, crystals about 5–15 mm

41809013.700 21857036.200 860 35

L-1c/R Do 50L/13 Hydrothermally altered and mineralized volcaniclasticrock containing quartz veins

41809014.3500 2185703400 850 45

L-2/RT 2.1 Hydrothermally altered and mineralized volcaniclasticrock; the sample 2.1 represents a composite ofthree rock samples slightly differing in mineralogy:(a) hydrothermal quartz, scorodite, realgar, barite,Fe-hydroxide, (b) hydrothermal quartz, realgar,orpiment, scorodite, sericite, Fe-hydroxides, and (c)hydrothermal quartz from a vein with quartzcrystals about 10–20 mm in diameter

41808040.500 2185703700 895 45

L-3a/RML 3.2/26

6/2-16/2-26/9

All the samples represent mineralized and hydrother-mally altered volcaniclastic rocks with similarmineralogyHydrothermal quartz, sericite, jarosite and Fe-hydroxide

41808034.700 21857029.400 850 20

L-3b/RML 3.1/25

5/9

The same as above 41808031.400 2185702700 830 25

L-3c/RML Do 1/13 The same as above 41808028.600 2185703000 845 35L-4/ADR 1.3/2

2.1/9The site is represented by silicified and mineralizedvolcaniclastic rockSample 1.3/2 is a mixture of hydrothermal quartz,sericite, marcasite, barite, jarosite, Fe-hydroxideSample 2.1/9is a vein composed of hydrothermalquartz with crystals about 5–8 mm in diameter

41808017.400 2185702700 820 15



suggests that the contribution of 36Cl from thermal neutron capture of 35Cl is only about 0.1% of that pro-duced from Ca [Phillips et al., 2001], and can therefore be neglected as well.

The concentrates of calcite/dolomite samples (ca. 150 g each) were sent to the PRIME Lab, where the finalsample preparation and the AMS measurements of 36Cl were carried out.2.3.3. Determinations of Cosmogenic 3He and 21Ne in Diopside, Quartz, and SanidineCosmogenic 21Ne concentrations were determined at GFZ Potsdam in one sanidine and six quartz sepa-rates, whereas the cosmogenic 3He concentration was assessed in one diopside separate. The samples werewrapped in Al foil and loaded in the sample carrousel above the extraction furnace, which was then bakedat 1008C for about one week. The noble gases were extracted by stepwise heating (400, 600, 800, and12008C for quartz; 600, 800, and 17508C for sanidine; 900 and 17508C for diopside) for at least 20 min each.Chemically active gases were removed by two Ti sponge getters and two SAES (ZrAl) getters, and He, Ne,and Ar-Kr-Xe were separated from each other by trapping in a cryogenic adsorber and subsequent sequen-tial release. Noble gas concentrations and isotopic compositions were determined in a VG5400 sector fieldmass spectrometer. Results were corrected for isobaric interferences, instrumental mass fractionation, andanalytical blanks. Further details about the analytical procedures can be found in Niedermann et al. [1997].

3. Results

3.1. Cosmogenic and Stable NuclidesThe results of 26Al measurements of 15 samples are shown in Table 2, those of 36Cl measurements arereported in Table 3. Stable Al concentrations in quartz from Allchar are uncommonly high, ranging from2520 to 3014 mg/g [Pavicevic et al., 2006]. This caused a decrease in the precision of 26Al/27Al measurements,which have a detection limit of �10215 for a stable Al concentration range of 100–200 mg/g.

The same sample set was analyzed for 10Be as well. However, compared to both 26Al and 21Ne, the 10Be con-centrations were apparently an order of magnitude higher, and consequently the nominal erosion rates cal-culated on the basis of these 10Be AMS measurements were considerably smaller than those obtained onthe basis of 21Ne, 26Al, and 36Cl concentrations. Because, in particular, a 10Be/21Ne ratio higher than the pro-duction ratio is physically impossible, such discrepancy suggests that our quartz purification procedure didnot successfully remove all meteoric 10Be from the grain surfaces, and therefore the 10Be results are notreported here.

Results of He and Ne analyses are shown in Tables 4 and 5, respectively. The 21Ne/20Ne ratios are onlyslightly above the atmospheric value (0.002959) in most heating steps of all quartz and sanidine samples.When plotted in a three-isotope diagram (Figures 2 and 3), all 400–8008C quartz data overlap with the ‘‘spal-lation line’’ [Niedermann et al., 1993] within error limits, indicating two-component mixing between atmos-pheric and cosmogenic Ne. Above 8008C, no cosmogenic Ne is released from quartz [Niedermann, 2002].Therefore, cosmogenic Ne in the quartz samples was calculated from the excesses over atmospheric com-position in the 400–8008C heating steps (Table 5); the small excesses observed in the 12008C steps are inter-preted to be nucleogenic Ne produced by the reaction 18O(a,n)21Ne, with a particles derived from U or Thdecay. In the sanidine sample, the 21Ne excess is barely significant at the 1r level (not significant at the 2rlevel), thus the total concentration of cosmogenic 21Ne based on the 6008C and 8008C steps [Kober et al.,2005] should be considered as an upper limit rather than a robust figure (Table 5). Similarly, cosmogenic3He in the diopside sample can only be reported as an upper limit, assuming that all 3He measured in thissample (87.3 6 3.9 3 105 at/g) is cosmogenic. Prior to the stepwise heating extraction, this sample hadbeen crushed in vacuo in order to characterize the composition of trapped noble gases. However, the Hereleased by this method was very small, <1% of that extracted by heating (Table 4), and showed an impre-cise 3He/4He ratio of (0.18 6 0.11) 3 1026. This indicates that the major 4He fraction in this sample is radio-genic and represents a component which resides in the crystal lattice and is thus not released by crushing.This interpretation is further supported by the isotopic composition of Ne (Table 4) and the ratio of excess21Ne to 4He of (4.22 6 0.19) 3 1028. The latter value is almost identical to the production ratio of nucleo-genic 21Ne to radiogenic 4He (4.5 3 1028) [Yatsevich and Honda, 1997]. Since the 3He/4He ratio of radiogenicHe is on the order of 1028, while the measured 3He/4He ratio was about 1.5 3 1027, it is reasonable toassume that the major fraction of 3He is cosmogenic. Nevertheless, we can only report cosmogenic 3He asan upper limit.



3.2. Estimates of the Erosion Rate at the Allchar Deposit3.2.1. Maximum Erosion Rates Based on Individual Cosmogenic NuclidesIn a first step, we have calculated maximum erosion rates for the 10 sampling locations in Allchar based onindividual cosmogenic nuclides. Table 6 presents the values of oblique and vertical erosion rates (seebelow) estimated by four individual cosmogenic nuclides in various monitoring minerals (total stoichiomet-ric and hydrothermal quartz, dolomite/calcite, sanidine, and diopside).

Production rates (resulting from spallation, slow muon capture, and fast muon-induced reactions) were cal-culated on the basis of geographical position of the sample locations (Table 1) using scaling factors accord-ing to Dunai [2001] for 26Al and 36Cl and Dunai [2000] for 3He and 21Ne. We assumed sea level and highlatitude (SLHL) production rates for spallation of 29.9 at/g SiO2/a for 26Al [Balco et al., 2008], 48.8 at/g Ca/afor 36Cl [Stone et al., 1996], 19.86 at/g SiO2/a for 21Ne [Goethals et al., 2009], and 124 at/g diopside/a for 3He[Goehring et al., 2010] (combined olivine and pyroxene production rate). The 21Ne production rate in sani-dine was assumed to be �1.5 times that in quartz [Kober et al., 2005]. This rough value is accurate enoughconsidering the large uncertainty of the cosmogenic 21Ne concentration in our sanidine sample Sa/08(Table 5). The contribution of muon-induced reactions to the total production was assumed to be zero for3He and 21Ne [Schoenbohm et al., 2004].

Two additional corrections have been applied to the acquired data for the calculation of erosion rates onsloped surfaces. First, we considered the dip angles of the sampled surfaces, i.e., we calculated shieldingfactors for sloped surfaces as given in equations (10) and (11) of Niedermann [2002] to account for theshielded fraction of cosmic rays. We then calculated erosion rates e using the relation

C5Pspexpð2 qh

KspÞ

k1 qeKsp

1Pmexpð2 qh

KmÞ

k1 qeKm

1Pf expð2 qh

KfÞ

k1 qeKf

(2)

Table 2. Acceleration Mass Spectrometry Measurements of 26Al in Quartz

SampleName

SampleMass

NativeAluminum

27AlCarrier 26Al/27Al

26Al/27AlStdDev 26Al 26Al StdDev

26AlStdDev

Location (g) (mg) (mg) (3 10215) (3 10215) (105 at/g SiO2) (105 at/g SiO2) (%)

L-1a/R 4a/2a 19.8075 1.51 0 180 0L-1b/R 4b/2 20.0555 1.30 83 72 1.2 1.0 87L-2/RT 2.1a 19.9885 56.99 0 0.8 0L-3a/RML 3.2/2 20.0278 40.97 13.8 6.8 6.3 3.1 49L-3a/RML 3.2/2b 24.697 50.0 1.037 17.9 6.3 8.2 2.9 35L-3a/RML 6 19.9119 58.93 10.3 3.7 6.8 2.5 36L-3a/RML 6/2 20.1449 40.25 19.6 5.1 8.8 2.3 26L-3a/RML 6/2b 24.671 53.6 11.4 3.4 5.5 1.7 30L-3a/RML 6/9 19.9863 58.47 8.4 3.3 6.7 1.2 20L-3b/RML 3.1/2 19.8943 45.43 8.5 2.7 4.3 1.4 32L-3b/RML 3.1/2b 24.769 57.3 1.116 12.6 4.2 6.7 2.2 33L-3b/RML 5 19.8894 59.94 6.2 3.6 4.2 2.4 58L-3b/RML 5/9 19.8293 59.30 7.0 2.8 4.8 0.8 17L-4/ADR 1.3/2a 20.0776 59.00 0 1.4 0L-4/ADR 2.1/9 19.8519 54.42 2.0 1.8 1.3 1.1 87

a26Al values below detection limit.bDuplicate measurements.

Table 3. Acceleration Mass Spectrometry Measurements of 36Cl

SampleName

SampleMass

Total Chloridein Rock

Total Chloridein rockStdDev 36Cl/Cla 36Cl/Cl StdDev 36Cl 36Cl StdDev

Location (g) (ppm) (ppm) (3 10215) (3 10215)(104 at/g ofthe Sample)

(104 at/g ofthe Sample)

L-1c/R Do 50L/13 14.6023 20.8 0.6 34.0 1.4 3.70 0.31L-3c/RML Do 1/13 13.1609 bdlb 54.3 2.7 5.23 0.44L-8/CD Do 8/13 14.7459 bdlb 32.3 1.6 2.16 0.30

aCl in denominator is total Cl (Cl in the rock 1 Cl added by dilution spike).bbdl—below detection limit.



where Ksp 5 160 g/cm2, Km 5 1510 g/cm2, and Kf 5 4320 g/cm2 are the attenuation lengths for spallation,muon capture, and production induced by fast muons, respectively, q 5 2.71 g/cm3 is adopted for the den-sity of the studied rocks, Psp, Pm, and Pf are the (local) production rates on the sloped surface (see above)through spallation, muon capture, and production by fast muons, respectively, C is the cosmogenic nuclideconcentration, h is the present depth of the samples, and k is the decay constant of radionuclides (k 5 0 for3He and 21Ne). Second, we applied an additional correction to the erosion rates according to equation (1) ofHermanns et al. [2004], which takes into account the reduced attenuation length beneath sloped surfacesand is based on earlier work of Dunne et al. [1999].

The resulting values determined in this way for 26Al in quartz agreed very well with those obtained by theCRONUS Earth online calculator (http://hess.ess.washington.edu/) [Xiuzeng et al., 2007; Balco et al., 2008]when the above corrections are included; the latter values are reported in Table 6. Finally, vertical erosionrates, i.e., downwearing rates of the overall horizontal landscape, were estimated by dividing those actingon inclined surfaces by the cosine of the dip angle (Table 6). We did not include any corrections for vegeta-tion or snow cover, because they are difficult to assess over geological timescales, but most probably insig-nificant. For example, Plug et al. [2007] estimate a 2.3% effect for temperate (Acadian) forest.3.2.2. 26Al-21Ne CombinationFigure 4 shows the ‘‘erosion island plot’’ for 21Ne/26Al versus 26Al. All data either overlap with the steadystate erosion island or lie above it in the complex exposure history field. Therefore, the combined 21Ne and26Al data can be explained by scenarios involving one or more periods of exposure (with or without erosion)

Figure 2. Neon three-isotope plot showing the stepwise heating extractions of three stoichiometric quartz samples from Allchar. Colors insymbols indicate heating steps: white—4008C, gray—6008C, red—8008C, and black—12008C. All 400–8008C data are consistent within 2rerror limits with the spallation line [Niedermann et al., 1993], i.e., with two-component mixtures of atmospheric and cosmogenic Ne. Contri-butions of 12008C steps, where no cosmogenic Ne is released any more [Niedermann, 2002], are small as reflected by large error bars. Themass fractionation line (mfl) is shown for reference. Note that unlike the data in Table 4, those plotted here do not include blank correc-tions, which would only increase the relative deviations from atmospheric composition as well as the uncertainties without changing thegeneral picture.



http://hess.ess.washington.edu

and one or more intermittent periods of burial. In case of samples 5/9 and 6/9 from locations L-3b/RML andL-3a/RML, respectively, the data would even allow for a simple exposure history with no erosion and no bur-ial. However, this is not reasonable in the geological context (see the next section). More likely, the positionsof the data points indicate a combination of erosion rates in the range from about 100 to 500 m/Ma andburial periods lasting for �0.5–3.5 Ma.

4. Discussion

4.1. Geological ConsiderationsGeological data indicate that during and particularly after volcanic activity, the rocks in Allchar underwenterosion and redeposition in adjacent terrain depressions. Because parts of the terrain, which were sampledfor this cosmogenic nuclide study, have likely undergone multiphase periods of burial by variably thick vol-caniclastic material, their exposure time must have been different as well.

The following geological and geochronological information should be taken into consideration before inter-preting the erosion rates in the area of Allchar:

1. The time span of volcanic activity was between 6.5 and 1.7 Ma [Boev, 1988].2. Alteration of the wall-rocks in the surroundings of the ore bodies Crven Dol and Centralni Deo, as well as

formation of thallium mineralization, commenced at 4.3 Ma.3. The maximal thickness of volcaniclastic material may have been up to several hundreds of meters.4. After the termination of volcanic activity, the volcaniclastic material was redeposited and accumulated in

the low level surroundings (depressions and river valleys).

Figure 3. Neon three-isotope plot showing the stepwise heating extractions of three hydrothermal quartz samples from Allchar. SeeFigure 2 for explanations.



5. There is no solid evidence about the existence of glaciers in the area of Allchar.6. The relevant erosion period lasted from the time of formation of the ore bodies until present day, and

was constantly influenced by exogenous processes.

All the above information about the Pliocene history of Allchar suggests that this area must not be consid-ered as a stationary rock complex. Rather than that its surface was constantly changing in the relevantperiod of 4.3 Ma.

Table 4. Results of Stepwise Heating He and Ne Analyses in Quartz, Sanidine, and Diopside Separates From Allcharc

4He 20Ne 3He/4He 22Ne/20Ne 21Ne/20NeSample, Location, Weight T (8C) 1028 10212 1026 1022 1022

5/9 quartza 400 0.00458 6 0.00028 11.23 6 0.42 5.2 6 3.1 10.15 6 0.20 0.313 6 0.021L-3b/RML 600 0.258 6 0.007 51.6 6 1.7 0.18 6 0.07 10.24 6 0.06 0.322 6 0.0110.70814 g 800 1.040 6 0.026 16.1 6 0.6 0.040 6 0.031 10.22 6 0.07 0.309 6 0.015

1200 0.316 6 0.008 1.99 6 0.24 0.13 6 0.07 9.87 6 0.43 0.523 6 0.040Total 1.619 6 0.028 80.9 6 1.9 0.094 6 0.028 10.21 6 0.05 0.323 6 0.008

2.1/9 quartza 400 0.0201 6 0.0006 36.5 6 1.2 1.6 6 0.8 10.38 6 0.06 0.313 6 0.011L-4/ADR 600 0.492 6 0.013 135.1 6 4.4 0.075 6 0.039 10.321 6 0.039 0.368 6 0.0070.71248 g 800 0.812 6 0.021 53.4 6 1.8 0.07 6 0.06 10.27 6 0.07 0.299 6 0.007

1200 0.284 6 0.007 12.91 6 0.47 0.19 6 0.13 10.67 6 0.16 0.326 6 0.011Total 1.608 6 0.025 237.9 6 4.9 0.112 6 0.039 10.338 6 0.030 0.342 6 0.005

6/9 quartza 400 0.00717 6 0.00038 8.53 6 0.33 200 (1600/–200) 10.12 6 0.18 0.320 6 0.026L-3a/RML 600 0.235 6 0.006 41.9 6 1.4 <1.1 10.28 6 0.09 0.333 6 0.0120.70726 g 800 1.398 6 0.035 22.3 6 0.8 0.033 6 0.022 10.28 6 0.09 0.303 6 0.011

1200 0.519 6 0.013 4.32 6 0.28 0.07 (10.08/–0.07) 10.02 6 0.27 0.344 6 0.042Total 2.159 6 0.038 77.1 6 1.7 0.7 (12.0/–0.7) 10.25 6 0.06 0.323 6 0.008

Sa/08 sanidine 600 0.539 6 0.014 96.9 6 3.5 0.12 (10.16/–0.12) 10.272 6 0.047 0.308 6 0.010L-1a/R 800 0.0929 6 0.0026 24.8 6 1.0 0.38 (10.41/–0.38) 10.45 6 0.08 0.307 6 0.0130.50028 g 1750 0.0217 6 0.0010 12.8 6 0.6 2.8 (12.9/–2.8) 11.16 6 0.23 0.321 6 0.015

Total 0.654 6 0.014 134.5 6 3.7 0.25 6 0.18 10.389 6 0.043 0.309 6 0.0084b/2 quartzb 400 0.1518 6 0.0047 32.2 6 1.3 0.43 6 0.36 10.34 6 0.12 0.297 6 0.013L-1b/R 600 0.428 6 0.013 147.9 6 4.6 0.18 6 0.14 10.32 6 0.07 0.301 6 0.0050.28214 g 800 0.0667 6 0.0023 69.2 6 2.5 0.6 6 0.6 10.37 6 0.08 0.308 6 0.012

1200 0.0018 (10.0020/20.0018) 4.8 6 1.0 4 (111/–4) 10.58 6 0.42 0.33 6 0.05Total 0.648 6 0.014 254 6 6 0.29 6 0.14 10.34 6 0.05 0.303 6 0.005

2.1 quartzb 400 0.0534 6 0.0019 59.2 6 2.0 0.6 (11.3/–0.6) 10.21 6 0.07 0.302 6 0.013L-2/RT 600 0.0272 6 0.0013 84.9 6 2.7 1.3 6 1.0 10.132 6 0.042 0.311 6 0.0060.29022 g 800 0.0046 6 0.0009 13.5 6 1.1 10 6 8 10.33 6 0.18 0.329 6 0.034

1200 0.0005 (10.0009/20.0005) 1.9 6 1.1 50 (190/–50) 8.2 6 1.2 0.34 6 0.07Total 0.0857 6 0.0026 159.5 6 3.7 1.6 6 1.2 10.155 6 0.042 0.310 6 0.007

2.1/9 quartzb 400 0.0795 6 0.0025 48.3 6 1.6 1.4 (13.5/–1.4) 10.30 6 0.07 0.3503 6 0.0047L-4/ADR 600 0.219 6 0.007 43.5 6 1.5 0.19 (10.37/–0.19) 10.25 6 0.10 0.307 6 0.0060.33898 g 800 0.1228 6 0.0039 1.3 6 0.8 0.37 6 0.34 11.1 6 1.0 0.45 6 0.12

1200 0.0150 6 0.0008 <0.23 1.5 (11.8/–1.5) - -Total 0.436 6 0.008 93.1 6 2.4 0.5 (10.7/–0.5) 10.29 6 0.06 0.3315 6 0.0041

Di/09 diopside 900 204 6 5 69.9 6 2.6 0.143 6 0.006 10.89 6 0.11 0.346 6 0.010L-7/KR 1750 14.93 6 0.38 6.8 6 1.2 0.222 6 0.037 13.6 6 0.7 1.13 6 0.150.37090 g

Total 219 6 5 76.7 6 2.9 0.148 6 0.006 11.13 6 0.12 0.416 6 0.0190.37852 g Crushed 1.348 6 0.034 37.9 6 1.2 0.18 6 0.11 10.35 6 0.12 0.298 6 0.013

aTotal stoichiometric quartz.bHydrothermal quartz.cNoble gas concentrations are in units of cm3 STP/g, error limits are 1r.

Table 5. 21Ne Excesses (Relative to Atmospheric Isotopic Composition, in Units of 105 at/g) as Determined by Stepwise Heating in Total Stoichiometric Quartz (*), Hydrothermal Quartz(**), and Sanidine (Sample Sa/08)a

Sample/Temp. (8C) 2.1/9* 5/9* 6/9* 4b/2** 2.1** 2.1/9** Sa/08

4008C 1.7 6 1.1 0.52 (10.61/–0.52) 0.55 (10.57/–0.55) 0.1 (11.1/–0.1) 1.0 (12.1/–1.0) 7.06 6 0.636008C 26.3 6 2.4 3.7 6 1.5 4.2 6 1.3 2.1 6 2.0 3.3 6 1.3 1.32 6 0.65 3.2 6 2.68008C 0.5 (11.0/–0.5) 0.57 (10.61/–0.57) 0.39 (10.64/–0.39) 2.2 6 2.1 1.2 (11.3/–1.2) 0.52 6 0.26 0.78 (10.85/–0.78)

12008C 1.04 6 0.37 1.22 6 0.17 0.56 6 0.49 0.39 (10.68/–0.39) 0.22 (10.30/–0.22) 0.76 6 0.18 0.86b 6 0.50Total �8008C 28.5 (12.8/–2.7) 4.8 6 1.7 5.1 (11.6/–1.5) 4.4 (13.1/–2.9) 5.5 (12.8/–2.0) 8.90 6 0.94 4.0 6 2.7

aTotal cosmogenic21Ne concentrations are calculated as the sums of the excesses in the �8008C steps. Error limits are 1r.b17508C step.



4.2. Episodes of Exposure and BurialAny stable geological surface that is exposed to cosmic rays accumulates cosmogenic nuclides. In principle,two major cases can be elaborated, namely, eroded and noneroded surfaces. In the Allchar case, we con-sider an eroded surface at various locations that are characterized by variable lithology. We may assumethat for a comparably short time se (up to a few thousand years), the erosion rate e was constant [e.g., Lal,1991; Niedermann, 2002]. If we calculate apparent exposure ages se 5 C21/P21 (where C21 is the 21Ne concen-tration and P21 its production rate) for different Allchar locations on the basis of 21Ne concentrations, postu-lating that during the last 100 ka the area of Allchar was neither covered by glaciers nor influenced by anyvolcanic activity (see section 4.1), then 9 ka� se� 93 ka. The high value of 93 ka is an exception; most val-ues being around 20 ka or less. For surfaces undergoing erosion, se corresponds to the characteristic erosiontimescale, i.e., the time it takes to erode the thickness of rock equivalent to one attenuation length. This isthe timescale over which our measurements integrate. As noted earlier, the ‘‘individual-nuclide’’ erosionrate values given in Table 6 must be regarded as upper limits, because any deviation from the assumed‘‘simple exposure history’’ will cause an overestimate of the mean erosion rate over the relevant time period.However, in exposure scenarios involving burial the true erosion rate during sufficiently short periods whenthe surface was not covered may nevertheless have been higher than the calculated ‘‘maximum erosionrate’’ for the whole period, because burial (or sedimentation) is equivalent to ‘‘negative erosion’’ and thusreduces the mean. As it is clearly not possible to apply a linear erosion model (i.e., a steady state irradiationhistory) for the whole period between the Pliocene (5.33–2.58 Ma) and Holocene (the last 11.5 ka) at Allchar,more complex scenarios of the exposure to cosmic rays must be considered.

Given that volcanic activity in the Allchar region lasted from 6.5 to �1.7 Ma, it can be supposed that it couldhave caused fast deposition of primary pyroclastic or volcaniclastic (redeposited) material. Such rapid burialof the surface rocks could have stopped the neutron-controlled production of cosmogenic nuclides for aslong as 4 Ma. This time period corresponds to almost six half-lives of 26Al. In other words, during the time ofburial, the ratio 21Ne/26Al would have increased �60 times. However, in the last Pleistocene/Holoceneepochs, from 200 to 300 ka to present, new phases of erosion and redeposition into adjacent depressionsand river valleys may have occurred. During that time surface rocks would have been again exposed to cos-mic radiation and this would have decreased the 21Ne/26Al ratio again. The erosion island plot for 21Ne and26Al (Figure 4) is in agreement with such a supposed geological history of Allchar. The high 21Ne/26Al ratiosimply a relatively long time of sedimentation (high duration of burial), some 2–3 Ma at the locations L-1b/R

Table 6. Oblique and Vertical Erosion Rates as Determined by 26Al, 36 Cl, 21Ne, 3He at Different Locations in Allchara

LocationSampleName

26Al (105 at/g SiO2)

Erosionrate (m/Ma)

Vert. erosionrate (m/Ma)

36Cl(105 at/g)

Erosionrate (m/Ma)


21Ne (105 at/g SiO2)

Erosionrate (m/Ma)


3He (105 at/g diops.)

Erosionrate (m/Ma)


L-7/KR Di/09 �95 �15 �15L-8 CD Do 8/13 2.16 365 387

(0.30) (186/275) (192/280)L-1a/R Sa/08 4.0b 59 69

(2.7) (1123/224) (1145/228)L-1b/R 4b/2 1.2 331 400 4.4 42 51

(1.0) (11700/2150) (12000/2180) (13.1/22.9) (181/217) (199/221)L-1c/R Do 50L/13 3.70 210 300

(0.31) (155/252) (178/274)L-2/RT 2.1 5.5 29 41

(12.8/22.0) (117/210) (124/214)

L-3a/RML

3.2/2; 6.7 63 67 5.1 37 393.2/2c;6;6/2-1;6/2-2; 6/9

(1.2) (115/211) (116/212) (11.6/21.5) (115/29) (116.0/29.0)

L-3b/RML3.1/2; 4.8 85 94 4.8 37 41

3.1/2c;5;5/9

(0.8) (119/214) (121/216) (1.7) (120/210) (122/211)

L-3c/RML Do 1/13 5.23 135 164(0.44) (141/236) (150/244)

L-4/ADR 1.3/2; 1.3 332 345 8.9 21.2 22(1.1) (11800/2160) (11860/2165) (0.9) (12.5/22.0) (13/22)

aAverage values are shown where several samples were analyzed for the same nuclide.b105 at/g sanidine.cDuplicate measurements.



and L-4/ADR and probably several hundreds of ka at L-3a/RML and L-3b/RML. A smaller burial duration atthe latter locations is reasonable because they are closer to the mountains and may thus have been uncov-ered earlier.

In a model scenario involving a single burial episode, the stable cosmogenic nuclides 21Ne and 3He fromthe analyzed samples originate from two periods: a preburial time, which lasted from the quartz crystalliza-tion to the moment when accumulation of volcaniclastic material occurred, and a postburial time, whichstarted when the cover was removed and lasted until present time. In contrast to 21Ne and 3He, the majorfractions of 26Al and 36Cl would have been produced during the last period of exposure, because 26Al and36Cl that was present before burial decayed almost completely during the long duration of burial. This iswhy the ratio 21Ne/26Al is higher than in case of a ‘‘simple exposure history,’’ i.e., when assuming a steadystate erosion model. Therefore, 26Al and 36Cl may yield reasonable estimates of the erosion rate prevailingin the postburial episode, i.e., over the last few tens of ka. Of course, the data do not allow us to infer anexposure history involving just one burial period. There may have been several successions of exposure andburial, but nevertheless, it is likely that the 26Al and 36Cl concentrations are most closely representing theerosion rate during the most recent exposure period.

4.3. Vertical Erosion Rates in Allchar Based on TCNAs already mentioned above, the main purpose of this study is to determine vertical erosion rates in a1.5 km long and 0.3 km wide, north-south elongated area of the Allchar deposit (see Figure 1). Because thisarea underwent a complex geological history, we applied different nuclides (both radioactive—26Al and36Cl and stable—3He and 21Ne) on different monitoring minerals (magmatic and hydrothermal quartz, dolo-mite/calcite, diopside, and sanidine) in order to acquire relevant information about prevailing erosion rates.The results are summarized in Table 6, where vertical erosion rate estimates are given in m/Ma for all theapplied cosmogenic nuclides.

The results show accordance in the values obtained by applying 26Al, 36Cl, 3He, and 21Ne. Hence, at samplelocations L-2/RT, L-3a/RML, L-3b/RML, and L-4/ADR (except for 26Al) the obtained erosion rate values varybetween several tens of m/Ma to hundreds of m/Ma. These values are similar to those from earlier AMSmeasurements, albeit on a considerably smaller number of samples. The erosion rates determined earlierbased on 26Al at the locations L-3a/RML and L-3b/RML are 49 and 59 m/Ma, respectively, whereas based on

Figure 4. Plot of 21Ne/26Al versus 26Al concentrations (scaled to sea level and high latitude) showing data from four Allchar locations. Mostdata lie in the burial field, indicating a combination of cosmic-ray exposure and erosion with intermittent burial periods. See text for dis-cussion. Graph was drawn in CosmoCalc version 1.8 [Vermeesch, 2007].



53Mn a rate of �40 m/Ma at the location closest to L-1a/R was estimated [Pavicevic et al., 2010]. However,the 26Al data from the localities L-1b/R, L-1c/R, L-4/ADR, and L-8/CD imply remarkably higher erosion ratevalues, which may potentially be explained by abrupt exhumations (sudden exposure to cosmic radiation),which must have occurred in the recent geological past in this part of Allchar. In spite of the mentioned dif-ferences, it can be postulated that a conservative lower limit to the prevailing erosion rates in the Allchararea is between 50 and 100 m/Ma. This range enables us to estimate vertical erosion rate values for theimmediate locations of the ore bodies of Crven Dol and Centralni Deo.

4.4. Paleo-Depth Estimates of the Crven Dol and Centralni Deo Ore BodiesA principle factor for the success of the LOREX Project is relevant information about the erosion rate of therock in the Allchar area for the last 4.3 Ma, which is the geological age of the Allchar deposit. Namely, incase of very low erosion rates, a small ratio of the experimentally determined total concentration of 205Pb inlorandite, i.e., (N205Pb)exp in equation (1), and the concentration of 205Pb produced by cosmic irradiationand other background processes, i.e., (N205Pb)B in equation (1), could potentially harm the project feasibility[Jelenkovic and Pavicevic, 1990]. The ratio (N205Pb)exp/(N205Pb)B in lorandite is predominantly depth-dependent, which means that it depends on the average depth of the lorandite from the time when themineralization precipitated to the present day [Pavicevic et al., 2010]:

dp5dtoday112

e � T (3)

where dtoday is the present-day depth of the lorandite ore and e is the average erosion rate for the timeperiod T 5 4.3 Ma. Although this is a simplified relation, neither taking into account the complex depthdependence of cosmogenic nuclide production by spallation and muon interactions nor the very probablevariations of the erosion rate in the past, it is sufficient for a first-order estimate.

Pavicevic et al. [2012] reported an estimate of the present amount of 205Pb produced in lorandite due tosolar neutrinos and background reactions in the last 4.3 Ma as a function of the paleo-depth of the lorandite[Pavicevic et al., 2012, Figure 2]. Furthermore, it is straightforward to estimate the present-day depth of thelorandite, taking into account the available information about mining horizons and geological boreholes inthe ore bodies Crven Dol and Centralni Deo. However, assessing the depth of the eroded layer (equation(3)) by applying TCN is a delicate task, especially bearing in mind that the Allchar area underwent a complexgeological evolution (see sections 4.1 and 4.2).

As argued above, the erosion rate values that we obtained by applying the radioactive nuclides 26Al and36Cl (Table 6) are valid only for the last geological past—essentially the Holocene. On the other hand,the concentrations of cosmogenic 21Ne and 3He (Table 6) reflect a combination of erosion and burialduring the total geological history of the Allchar area, from the Lower Pliocene to the present day. There-fore, the estimates of erosion rate obtained from the stable nuclides 21Ne and 3He can be considered ascloser to ‘‘long-term average values’’ than those that are calculated on the basis of the radioactivenuclides 26Al and 36Cl. Consequently, the erosion rate estimates based on stable nuclides and thoseobtained by radioactive nuclides are considered minimal and maximal erosion rates in the Allchar area,respectively.

On the basis of the erosion rate values reported in Table 6, equation (3) and the values of dtoday (28 m forthe ore body Crven Dol 1, 103 m for the shallower part of the ore body Centralni Deo 3, and 140 m for thedeeper part of the ore body Centralni Deo 4) [Pavicevic et al., 2012, Table 1], we estimate a maximum paleo-depth value for the ore bodyCrvenDol(d1) as well as minimum and maximum values for the ore bodyCen-tralniDeo (d3 and d4) are as follows:

d1Crv:Dmax 5 860m 1200m=2180m

d3Cen:Dmin 5 250m 1310m=260m; d3Cen:D

max 5750m 1170m=2160m

d4Cen:Dmin 5 290m 1310m=260m; d4Cen:D

max 5 790m 1170m=2160m

These values should be multiplied by a factor of 2.71 to obtain paleo-depths in units of meter water equiva-lent (mwe).



At the Crven Dol locality, we did not have samples in which the stable nuclides 3He or 21Ne could be meas-ured, and this is the reason why we do not have a lower limit estimate of the paleo-depth there. In spite ofthis, these data provide a reasonable basis for estimating the pp-neutrino and fast muon contributionsto205Pb production.

5. Conclusions

Based on the combined analysis of 26Al and 21Ne in a subset of our samples (Figure 4), it can be concludedthat the Allchar area underwent a complicated geological history and that cosmic irradiation of the surfacerocks reflects erosion rates ranging from �100 to 500 m/Ma and burial ages between �500 ka and 3 Ma.

By estimating the maximum erosion rates at 10 different localities of Allchar (within an area of around0.3 km 3 1.5 km), applying two radioactive (26Al and 36Cl) and two stable (3He and 21Ne) nuclides in quartz,calcite/dolomite, sanidine, and diopside as monitoring minerals, we established that 85% of the valuesrange from several tens of m/Ma to 350 m/Ma, depending on the location.

The obtained range of erosion rates based on stable and radioactive nuclides enables us to estimate reason-able limits for the vertical erosion rate values at the locations of the main lorandite ore bodies Crven Doland Centralni Deo. On this basis, we estimate that the minimum and maximum paleo-depths of the orebody Centralni Deo (averaged from 4.3 Ma to present) are 250–290 and 750–790 m, respectively. In relationwith this, we further estimate a maximum paleo-depth for the ore body Crven Dol (over the same geologi-cal age) of around 860 m.

These values allow us to determine the ‘‘upper and lower limit’’ of the contributions of 205Pb (number ofatoms 205Pb/g of lorandite), which have been caused by interaction of fast cosmic-ray muons over 4.3 Ma inAllchar lorandite. As a result, the ratio of signal (contribution of 205Pb by pp-neutrinos) to background (con-tribution of 205Pb by fast muons) is in the interval from 1:1 to 4:1, which implies a 1r error of the mean neu-trino flux d 5 20% to 30%, lower than the value estimated in Pavicevic et al.[2012].

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AcknowledgmentsThe original data used in this studycan be made available by thecorresponding author, up on request.This study is directly supported by theFWF—Wien grant P 25084 N27. Wealso appreciate the continuoussupport of PRIME Lab, PurdueUniversity for their work in thechemical preparation of targetsamples from quartz, AMSmeasurements of 26Al and 36Cl and fortheir very helpful information abouterosion rate calculations. We are verymuch indebted to G. Tippelt(University of Salzburg) for the X-raydiffraction measurement of rocksamples and quartz separates and toE. Schnabel (GFZ Potsdam) forperforming the noble gas analyses.M.K.P. is very thankful to the Universityof Salzburg for its hospitality andfinancial support. V.C. thanks theproject 176016 of the Serbian Ministryof Education, Science, andTechnological Development.



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Erosion rate study at the Allchar deposit (Macedonia) based ...tively. In this context, the knowledge of the erosion rate of the overburden rocks since the formation of lor-andite,